custom REST endpoint that returns all categories for posts in WordPress:

endpoint that returns all categories

<?php

/**
 * Plugin Name:Ali Custom API 
 * Plugin URI:
 * Description: Crushing 
 * Version: 1.0
 * Author: Ali
 * Author URI: 
 */

function get_all_categories( $request ) {
    $categories = get_categories( array(
        'taxonomy' => 'category',
        'hide_empty' => false,
    ) );

    $data = array();

    foreach ( $categories as $category ) {
        $data[] = array(
            'id' => $category->term_id,
            'name' => $category->name,
            'slug' => $category->slug,
            '' => $category->description,
            'count' => $category->count,
        );
    }

    return new WP_REST_Response( $data, 200 );
}
add_action( 'rest_api_init', 'register_custom_endpoints' );

function register_custom_endpoints() {
    register_rest_route( 'myplugin/v1', '/categories/', array(
        'methods' => 'GET',
        'callback' => 'get_all_categories',
    ) );
}